Alexa.com isn't the greatest, but it can still be used for certain niches.
Now I hear all the time "Alex.com is a pointless metric to evaluate your website with" but I don't use it for MY website
The only reason I'll use alexa is because I'm trying to find good advertising spots. If you're like me and you're in the SEO and Online Marketing niche, it's a good thing to check when you are thinking about buying an ad spot on a random website.
Why? Because the people who are more inclined to download the alexa toolbar are the ones who own websites If they own websites then you can sell to them
Now you're saying "Well not everyone will download the alexa toolbar and some websites will be better than others to advertise on even if their alexa rank isn't as good" that's true, but use every metric you can before you purchase an ad spot. I'd rather advertise on a website that has a 10k alexa rank than one with a 500k or higher alexa rank. They could be getting the same amount of traffic, but the one with the 10k ranking will be better for SEO and Marketing companies to advertise on since the users are more likely to purchase your services.
I've done plenty of research on websites to try and figure out if they are worth the money to put up an ad. Some pan out very well and others are complete flops. But the ones with the higher alexa rankings tend to be a little better for me than the others
Now this really only applies to anyone like me who is in the same field and not someone in the plumbing, comedy, video game, and any other niche. Well you could probably do this with web design, but you may not get as many sales